How do I parse custom tags in an HTML file? eg:<mytag name="aaa">xxx</mytag>

Custom tags that are placed within JSP pages are parsed by the JSP container automatically, provided of course that they are setup and defined correctly.

If you have a static piece of HTML containing some common information and custom tags (e.g. a HTML header), this can be statically included in a JSP using the include directive.

For more information about custom tags, take a look at another question explaining how to create custom tags and get them working in Tomcat.